Spectrometer attachments and phosphorescence decay measurement

ABSTRACT

A spectrometry instrument with exchangeable accessories ( 34, 48, 50, 52 ) providing, for example, different sample presentation facilities. The accessories include a manually operable cam-lock facility ( 54, 68 ) for quick and easy attachment of an accessory to the instrument. The instrument also includes an electrical circuit ( 86-90 ), which is completed by a circuit portion ( 100 ) in an accessory when the accessory is attached to the instrument, for generating a unique identifying voltage ( 94 ) to thereby identify that accessory. This allows for automatic loading in a controlling computer of programs for setting up and running the instrument for measurement regimes using that accessory. The spectrometry instrument is preferably a spectrophotometer used for phosphorescence decay measurements in which sequential phosphorescence emission measurements data from each of a number of excitation cycles applied to a sample are taken and then reassembled into a correct time sequence to define a phosphorescence decay characteristic for the sample, that is, measured data points from a second (and subsequent) excitation cycle are interleaved with those from a first excitation cycle. This significantly reduces the time for establishing a phosphorescence decay characteristic.

RELATED PATENTS

This application is a division of Ser. No. 09/831,244, filed Jul. 2,2001, now U.S. Pat. No. 6,657,720, which is the national stage ofPCT/AU00/01058, filed Sep. 6, 2000.

TECHNICAL FIELD

This invention relates to spectrometry instrumentation in general and inparticular examples to fluorescence, phosphorescence and luminescencespectrophotometry.

BACKGROUND

A fluorescence spectrophotometer usually comprises a flash light source,an excitation monochromator or filter, a sample cell containing a sampleto be analysed, an emission monochromator or filter, a photodetector andsignal processing electronics. A specific wavelength of light from theflash source, as selected by the excitation monochromator or filter, isdirected into the sample cell and resultant fluorescence light from thesample enters the emission monochromator or filter. A specificwavelength of the fluorescence light, as selected by the emissionmonochromatolr or filter, is directed onto the photodetector to producean electrical signal corresponding to the intensity of the fluorescentlight. Such an instrument may be arranged to make a fluorescence,phosphorescence or luminescence measurement. Fluorescence measurementsrelate to light which is emitted virtually immediately by a sample uponits exposure to the excitation light, whereas phosphorescencemeasurements relate to the light emitted from the sample a shortcharacteristic time after its exposure to the excitation light.Luminescence measurements are taken by measuring the emitted light froma sample without exposing the sample to excitation light. Suchmeasurements are used to characterise substances, with fluorescencemeasurements in particular having wide application in the biotechnicalfield for characterising DNA and other proteins, for example usingfluorofors.

It is known in spectrometry instruments in general, and inspectrophotometers for fluorescence, phosphorescence and luminescencemeasurements, to provide exchangeable accessories. Generally these mayprovide different sample presentation facilities, for example a liquidsample presentation accessory may be exchanged for one which providesfor presentation of a solid state sample. Different accessories may alsoprovide for temperature control of samples via Peltier, Dewar or othercryostat devices, successive feeding of multiple samples to a readinglocation, or multiple sample carriers such as a well plate and readertherefor.

In order not to compromise test results, it is important that theexchangeable accessories for a spectrometer be repeatably and accuratelylocatable on the instrument. Prior art arrangements for doing this,which involve screw threaded attachment of one part to another,generally do not facilitate rapid exchange of one accessory for another.

As described above, the capability to make phosphorescence measurements(that is, phosphorescence emission intensity versus time) is included insome fluorescence spectrophotometers. To collect phosphorescenceintensity versus time data that results from a short pulse of excitationlight, it is necessary to repetitively measure the emission intensity ata time short enough to adequately define the relationship. The capturingof a data point can be done relatively quickly via a sample and holdcircuit, however the measurement and digitisation of that data pointtypically takes a reasonable length of time. Such data conversion oftentakes longer than the required interval between successive measuredpoints. By way of example, adequate definition of the emission timerelationship may require measurement of the emission intensity at 1microsecond intervals yet the digitisation of a single emission datummay take, say, 19.5 microseconds. For this reason, the prior arttechnique is to use a sampling approach. In this arrangement, theexcitation light pulse is generated repetitively at a constant interval.The interval must be long enough for the emission from one pulse to havefallen substantially to zero before the next pulse is applied. Aftereach excitation pulse a single emission intensity is measured at acontrolled time after the excitation pulse so as to give a single datumof the emission time relationship. For each successive cycle the timeinterval between the excitation and capturing of emission intensity ismodified so as to build up a complete picture of the overall emissionversus time relationship. In the example given for the first cycle thetime delay could be 1 microsecond. For the second cycle the time delaymay be 2 microseconds. For the third the delay will be 3 microsecondsand so on.

The problem with this approach is that the interval between excitationpulses must be long enough to allow the emission to die awaysubstantially to zero between one pulse and the next. At the same timemany cycles are needed to build up a comprehensive picture of theemission versus time relationship. The overall measurement is thus slow.For example, again referring to the above example of one microsecondintervals between data points, if data covering two milliseconds isdesired then 2000 data points will need to be collected. If the time forthe emission to substantially fall to zero is 10 milliseconds, it willtake 20 seconds to complete the 2000 measurement cycles.

SUMMARY OF THE INVENTION

According to a first aspect the present invention provides aspectrometry instrument and an exchangeable accessory therefor includinga manually operable mechanism for attaching the exchangeable accessoryto the instrument, the mechanism including a manually rotatable cammingmeans associated with one of the accessory or the instrument, a malemember associated with the other of the accessory or the instrument. Themale member having a camming surface which is engageable by the cammingmeans, wherein the accessory is positionable on the instrument in apredetermined location and the camming means is manually rotatable toengage the camming surface of the male member and thereby lock theaccessory on the instrument in the predetermined location.

In spectrometry instruments which have exchangeable accessories, itwould be advantageous if the instrument could detect if an accessory hasbeen attached and if so, to identify what accessory it is. Theadvantages of this include the instrument's set up and programming foruse with a particular accessory being able to be automaticallyestablished. Also for those accessories that include electricalcomponentry, such as stepper motors, it would be advantageous to detectthe presence of such a component.

According to a second aspect the present invention provides aspectrometry instrument including an electrical circuit for identifyinganyone of a plurality of exchangeable accessories which are connectableto the instrument, the electrical circuit including a voltage source andmeans for generating an identifying voltage therefrom, wherein eachaccessory includes at least one circuit element such that connection ofan accessory to the instrument alters the identifying voltage to a valuewhich is uniquely dependent upon the accessory which is connected to theinstrument.

The accessory recognition circuitry may be such that it recognises thepresence of an electric motor of an accessory. In this case a voltagedivider can be arranged to provide a logic high signal in the presenceof a motor by virtue of the motor winding completing a circuit betweenthe voltage source and the voltage divider. In the absence of the motor,the circuit is open and a logic low signal is derived from the voltagedivider.

Preferably the spectrometer includes circuitry for identifying anaccessory and further circuitry for determining the presence or absenceof an electric motor in that accessory.

For a spectrometer with a capacity to have a number of differentaccessories connected thereto at the same time, each connection socketfor each accessory may include accessory recognition circuitry as abovedescribed. In this arrangement, the signal line for the identifyingvoltage from each circuit may be connected to a multiplexer for input toa microprocessor of a computer.

In a third aspect the present invention provides a method and apparatusfor reducing the time for measuring a number of data points fordetermining a phosphorescence decay characteristic (that is,phosphorescence emission intensity versus time) of a sample.

According to this third aspect, there is provided a method ofdetermining a phosphorescence decay characteristic of a sample or atleast a portion thereof, including

i) exposing the sample to a first excitation hash of light,

ii) measuring the intensity of a decaying phosphorescence light signalfrom the sample caused by the first excitation flash at each of asequence of measurement points which commence a controlled time afterthe first excitation flash and are separated by controlled times,

iii) exposing the sample to a second excitation flash of light and

iv) measuring the decaying phosphorescence light signal from the samplecaused by the second excitation flash at each of a sequence ofmeasurement points which commence a controlled time after the secondexcitation flash and are separated by controlled times, wherein the timeinstants to the first and subsequent measurement points from the secondexcitation flash lie between the first and subsequent measurement pointsrespectively from the first excitation flash,

v) assembling the phosphorescence measurements into time sequence toproduce a phosphorescence decay (characteristic, or a portion thereof,for the sample.

The assembly of the phosphorescence measurements into time sequenceresults in the measured data points from the second excitation flashbeing interleaved with those from the first excitation flash.

In some cases as the phosphorescence emission from a sample decays, thetime interval between the data points which is required to adequatelydefine the phosphorescence characteristic becomes longer. The abovedescribed method, in relating to determining possibly only a portion ofa phosphorescence decay characteristic, recognises that after a certaintime, the necessary time interval between data points to adequatelydefine the characteristic may be so long as to be able to besequentially measured from the emission caused by one of the excitationflashes and not both. Thus the above described method may be appliedonly for determining an initial or any particular predetermined portionof a decay characteristic.

The time intervals in step (ii) established by the controlled times aregreater than the measurement and digitisation time. These intervals maybe controlled in the sense they are prior determined or computed duringthe data collection process (that is, they are computed “on the fly”from the time for measurement and digitisation of data). The timeintervals between measured data points may be uniform or vary from oneinterval to the next. Similarly, the time intervals in step (iv)established by the controlled times may be prior determined ordetermined by computation during the data collection process.

The method may be extended wherein further excitation flashes areinitiated and further phosphorescence emission intensity measurementstaken which result from each such further excitation flash, the furtherphosphorescence measurements for each such further excitation flashbeing taken at controlled times (ie., prior determined or computed timesas above described) such that each such further phosphorescencemeasurement can be interleaved between phosphorescence measurementsresulting from earlier excitation flashes. That is, steps (iii) and {iv)may be repeated as often as necessary until all required measured pointsare obtained.

According to this third aspect of the invention there is also providedapparatus for performing the above described method. This apparatuscomprises a spectrophotometer and means for controlling thespectrophotometer, said means for controlling being such as to acquiresequential phosphorescence emission measurements data from each of anumber of excitation cycles applied to a sample in the spectrophotometerand to assemble that data into a correct time sequence to define aphosphorescence decay characteristic, or a portion thereof, for thesample.

The means for controlling the spectrophotometer may be a suitablyprogrammed computer or a dedicated device or circuitry.

Preferably this apparatus includes a manually operable mechanism forattaching an exchangeable accessory as described herein above. Theapparatus also preferably includes an accessory recognition circuit asalso described hereinabove.

The following detailed description with reference to drawings isprovided to give a better understanding of the invention and to show howit may be carried into effect in all its aspects. This description andthe drawings are given by way of non-limiting example only and are notto be interpreted as limiting the generality of the precedingdescription.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 diagrammatically illustrates a spectrophotometer for measuringfluorescence or phosphorescence from a sample;

FIGS. 2 to 4 illustrate a manually operable mechanism for attaching anaccessory to a spectrophotometer;

FIGS. 5 and 6 illustrate accessory recognition circuitry for use in aspectrophotometer;

FIGS. 7A and 7B illustrate data acquisition circuits; and

FIG. 8 diagrammatically illustrates a phosphorescence measurementregime.

DESCRIPTION OF PREFERRED EMBODIMENTS

A fluorescence spectrophotometer, as diagrammatically illustrated inFIG. 1, includes a Xenon flash light source 20, the light 22 from whichis directed into an excitation monochromator 24. Light 26 of a selectedwavelength which exits monochromator 24 passes through a beam splitter28 to derive a reference beam 30 the intensity of which is measured by adetector 32. Excitation light 26 continues from beam splitter 28 andirradiates a sample in sample holder 34. The fluorescence (orphosphorescence) light 36 emitted by the sample traverses an emissionmonochromator 38, the light of a selected wavelength 40 of which theintensity is measured by detector 42. The emission monochromator isarranged to be off the axis of the excitation monochromator 24. Drivers44 and 46 for each of the monochromators 24, 38 respectively, allow forwavelength, filter and slit width selection.

Operation of the spectrophotometer is controlled by a computer or othermeans (not shown in FIG. 1) such that slit: widths and filters areselectable according to wavelength and controlled by stepper motorsallowing either manual or automatic selection. Slit selector is usercontrolled. The computer or other control means also controls the dataacquisition electronics (to be described below) and the manipulation ofthe data, notably for phosphorescence measurements, also to be describedbelow.

The detectors 32 and 42 are photomultiplier tubes. If the light tightsample compartment door of the instrument is opened allowing incidentlight to reach the photomultiplier tubes 32 and 42, the firmwarerecognises this overrange condition and causes a filter to be moved toblock the entry of the incident light into the photomultiplier tubesand/or reduce the EHT power supply. Another protection feature is thatmonochromators 24 and 38 include safety interlocks for preventing a zeroorder setting for slit widths greater than 5 nm. The instrument includesexchangeable accessories schematically represented at 48, 50 and 52.Such accessories generally provide for different samples and samplepresentation regimes and are thus exchangeable in relation to the sampleholder 34. The instrument may simultaneously have a number, for exampleup to four, different accessories connected thereto. All accessoriesrequire mechanical attachment to a sample compartment of the instrumentand in one aspect this invention provides a quick, simple and reliableattachment mechanism for this. The sample compartment of a spectrometeris an accessible space within the spectrometer wherein a sample isconveniently placed for the purpose of making spectrometricmeasurements. A sample compartment is typically provided with means tohold a sample in a precisely defined position with respect to the pathsof light beams in the spectrometer, and is provided with apertures forthe passage of said light beams. In another aspect the inventionprovides electrical means for detecting the presence or absence of anaccessory, and if an accessory is attached and plugged in, identifyingthat accessory so that appropriate software programmes for measurementregimes using that accessory may be automatically loaded. This savesuser time in that the user does not then have to search for the relevantprogrammes.

With reference to FIGS. 2-4, the mechanical attachment mechanism for anaccessory such as 48 comprises a manually rotatable camming means 54associated with the accessory 48. FIG. 2 shows an underneath view of thebase 56 of an accessory 48 on which is mounted for rotation a shaft 58for manually rotating the camming means 54 via a handle 60 (a platewhich is attachable to the base 56 for covering the shaft 58 has beenomitted from FIGS. 2 and 3). FIG. 3 shows a section of FIG. 2 on lineIII—III.

Ideally the camming means is rotated through less than 360° to attachthe accessory and more ideally its rotation is about 180°, Preferablythe camming means has a female form for receiving the male member, forexample it may be spherical or cylindrical with a recess formed thereinhaving a curved camming surface which interacts with the camming surfaceof the male member. The camming means is preferably locatedsubstantially centrally of a base of the accessory and is operable via ashaft which extends to a peripheral surface of the accessory for manualoperation. Preferably the shaft includes a handle or knob forfacilitating its manual rotation.

Preferably the male member is associated with and is biased in adirection towards the instrument such that, as the camming means and themale member become engaged, the male member is moved in a direction awayfrom the instrument against the bias. This ensures that when the cammingmeans and the male member are fully engaged to lock the accessory ontothe instrument, a positive holding force is maintained on the accessory.

Alternatively the camming means may be associated with the instrumentand the male member with the accessory.

Preferably the accessory and the instrument include a number ofcomplementary projections such as pins on and recesses in their facingsurfaces for establishing the predetermined location for the accessoryon the I instrument. Thus, as the camming means is rotated to engage themale member and draw the accessory towards the instrument, theprojections, which are preferably on the accessory, locate incomplementary recesses which are preferably in the instrument, to ensurethe correct location of the accessory on the instrument.

It will be appreciated that embodiments of the invention as describedabove and to be described in more detail below provide an easilymanually operable attachment mechanism which allows quick attachment andrelease of an accessory from a spectrometry instrument which ispreferably a spectrophotometer. This quick attachment and releaseadvantage of the invention is derived from the mechanism's use of asingle attachment point and the actual attachment being achieved by anapproximate half turn of the camming means via a prominently accessiblehandle, knob or the like.

The camming means 54 comprises a cylinder 62 within which a recess 64 isformed which provides a curved camming surface 66.

A male member 68 (see FIG. 3) is mounted in the base 70 of the samplecompartment of the instrument and includes an outer spherical form 72which is engageable by the camming means 54, specifically its cammingsurface 66, whereby rotation of the camming means 54 via handle 60causes its surface 66 to interact with spherical surface 72 of malemember 68 to draw the accessory base 56 into facing contact withinstrument base 70 and lock the accessory on the instrument in apredetermined location. The predetermined location is determined by therelative locations of the camming means 54 and the male member 68 and bycomplementary location means on an accessory and the instrument. Thesecomplementary location means may comprise protrusions 74 on theaccessory base 56 (only one of which is shown in FIGS. 2 and 3) whichare locatable in recesses 76 in the instrument base 70. A convenient andpreferred mechanism for spatially locating the accessory in the samplecompartment is to use a kinetic mount. This consists of three protrudingpegs on either the instrument or the accessory The first peg engages ina hole in the mating surface and thereby accurately locates one point ofthe accessory to the instrument. Height control may be achieved eitherby the peg resting on the bottom of a blind hole or a shoulder on thepeg resting on the top of the hole. The second peg locates in a slot inthe mating surface whose centre line passes through the centre of thepreviously mentioned hole. It uses similar means of height control asfor the first peg. This controls angular position of the accessory withreference to the first location point. The third peg rests on a plate onthe mating surface.

The spherical form 72 of the male member 68 is at the end of a stem 78mounted in a sleeve 80 and biased inwardly relative thereto by a spring82. The sleeve 80 is screw-threaded at a lower 01″ inner end 84 forattachment in an aperture 86 in the base. Thus as the camnning means 54engages the male member 68 and is rotated relative thereto the spring 82acts to bias the spherical form 72 downwardly towards the base 70 of theinstrument. This ensures that when the camming means 54 and the malemember 68 (specifically the surfaces 66 and 72) are fully engaged tolock the accessory 48 onto the instrument, a positive holding forcemaintained on the accessory.

The accessory 48 is releasable simply by reversely manually rotating thehandle 60 to release the spherical form 72 from the camming surface 66and lifting the accessory away.

The spectrophotometer includes a number of sockets, for example four, inits sample compartment for receiving plugs on the accessories, that is,each accessory has a plug which is receivable in anyone of the foursockets. An accessory recognition circuit in the spectrophotometerincludes a voltage source 86 (see FIG. 5) the negative side of which isconnected to ground and the positive to a means for generating anidentifying voltage in the form of a voltage divider comprisingresistors 88 and 90. The series connection of the source 86 andresistors 88 and 90 is connected to a dedicated pin 92 of a socket inthe sample compartment. A signal line 94 is connected between theresistors 88 and 90 and a multiplexer 96, and then to an analog todigital converter 98 and a microprocessor (not shown) for reading thedata and controlling operations. An identifying voltage for an accessoryis read via signal line 94. If there is no accessory present, thiscircuit is open and the voltage of source 86 is read on line 94.

Preferably the means for generating an identifying voltage is a voltagedivider and this together with the voltage source provide an openelectrical circuit such that in the absence of an accessory theidentifying voltage floats to the voltage of the voltage source, therebyidentifying the absence of an accessory. Preferably each accessoryprovides a circuit element for completing the electrical circuit of thespectrophotometer when connected thereto. The circuit element of eachaccessory is different such that when it completes the circuit includingthe voltage source and the voltage divider of the spectrometer, itcauses the identifying voltage to change to a value which is unique forthat accessory. The identifying voltage which is generated is read by amicroprocessor which identifies the particular accessory, or absence ofan accessory I connected to the spectrometer, which is preferably aspectrophotometer.

The circuit element of an accessory, may simply provide a link whichconnects to ground, or a particular voltage of the instrument, eg. +5V,+12V, +15V or −15V, depending on the accessory. This arrangement can beused for accessories which do not include their own electronics. Foraccessories which do include their own electronics and thus a circuitboard and a plurality of circuit elements, a resistor may be includedwhich connects between the circuit of the instrument and a connection toground. +5V, +12V, +15V or −15V. It will be evident that a number ofcircuit combinations are possible to provide for a number of differentaccessories. For example, a circuit element of an accessory in the formof a link that connects to ground, +5V, +12V, +15V or −15V gives 5combinations. That is, it gives the possibility of generating fiveunique voltages and thereby the identification of five differentaccessories.

Connection of an accessory to the socket may provide a circuit elementin the form of a link (not shown) to ground, or to an analog voltage,say +5 volts, +12 volts, +15 volts or −15 volts on other pirls of thesocket, depending on the particular accessory. When such a link is made,the voltage appearing on line 94 will alter to a value which is uniquelydependent upon the particular link established by that accessory. Thusthe voltage signal on line 94 can be used by the microprocessor toidentify a particular accessory. Such a link for completing the circuitof the instrument is suitable for accessories which do not include theirown circuitry. Furthermore the possibility of the link connecting toground, +5 volts, +12 volts, +15 volts or −15 volts provides fivecombinations, that is, it allows the identification of five differentaccessories.

The multiplexer includes four signal inputs, one from each of a circuitsuch as is illustrated associated with each of 1 the four accessorysockets.

Alternatively where an accessory does include its own circuitry, aresistor 100 may be added which is connectable, via the plugging in ofan accessory to one of the instrument sockets, to ground or ˜3 supplyvoltage such as +5 volts, +12 volts, +15 volts or −15 volts at pin 102.This will also alter the voltage on signal line 94 to a unique value forthe particular accessory concerned. This allows more combinations forthe identifying voltages 94 than the previous arrangement of using onlya link. Alternatively the resistor 100 may be connected to or replacedby a programmable voltage source to allow for re-configurableaccessories.

For an accessory with a stepper motor, the recognition circuitry maycomprise a pull-up resistor 104 (see FIG. 6) connected between a voltagesource 106 (eg. 12V) of the instrument and a pin 108 of the accessorysocket. A voltage divider comprising resistors 110, 112 is connectedbetween another pin 114 of the socket and ground. A signal line 116 isconnected between the voltage divider resistors 110, 112. Motor drivers118, 120 are connected to the pins 108, 114. FIG. 6 shows a motor of anaccessory having a winding 122 connected across the pins 108, 114. Onpower up the motor drivers 118, 120 are disabled and pull up resistor104 and voltage divider 110-112 generate a “motor present” signal, thatis, if there is a motor winding connected across pins 108, 114 a currentflows through the voltage divider 110-112 which generates a logic highsignal (indicating “motor present”) which is read by the microprocessor(not shown) to which signal line 116 leads. If a motor is not present, alogic low signal on line 116 is read by the microprocessor.

The plug of an accessory may be arranged on the accessory such that itautomatically mates with a socket of the instrument as the accessory isattached thereon via a mechanical attachment mechanism as describedhereinabove. Thus the one action of attaching an accessory mayautomatically establish its electrical connection to the instrument andcompletion of the recognition circuitry and the possible consequentialautomatic loading of programmes.

A data acquisition circuit of an instrument as in FIG. 1 isdiagrammatically illustrated in FIG. 7A. This circuit comprises anamplification stage 124 connected to a detector 42 as in FIG. 1. Theoutput of the amplification stage 124 is connected to a sample and holdcircuit 126, the output of which is i connected to an analog to digitalconverter 128 which supplies the data to a microprocessor of a computer130. The instrument is computer controlled and this is represented byline 132 (alternatively the instrument may be controlled by a dedicateddevice or circuitry). Multiple channel data acquisition circuits may beprovided, or as illustrated, separate sample and hold circuits 134, 136etc, each followed by an analog to digital converter {not shown) may beconnected between the amplification stage 124 and the computer 130. FIG.7B illustrates a modification of the FIG. 7A circuit, namely theaddition of control circuitry 138 which receives a signal on line 140from an AID converter 128 indicating that a conversion is complete andsending a signal on line 142 to a sample and hold circuit 126, 134, 136to start another conversion. That is, the additional circuitry 138-142determines the measurement time dynamically, typically initiating thenext conversion whenever one of the conversion circuits becomes idle.

Use of a fluorescence spectrophotometer as in FIG. 1 having a dataacquisition circuit as in FIG. 7 and which may have either or both ofthe accessory attachment and accessory recognition features describedhereinabove, for making phosphorescence measurements will now bedescribed to exemplify the third aspect of the invention.

A problem with a data acquisition circuit such as that of FIG. 7A or 7Bis that the gate time for the digitisation and reading of a sample datapoint from the sample and hold circuit 126 by the analog to digitalconverter 128 and microprocessor 130 usually exceeds the time space inbetween the data points which is necessary to adequately define thephosphorescence decay characteristic of the sample. That is, theelectronics is not fast enough to convert all of the necessary data,hence a relatively high number of flash and read cycles have to beperformed, with only one data point being collected for each cycle.

The controlled times between the phosphorescence emission measurementpoints for a particular excitation flash may be equal, with the time tothe first measurement point resulting from the second excitation flashbeing different to and preferably longer than the time to the firstmeasurement point resulting from the first excitation flash. Continuingwith this sequence, the time to the first measurement point resultingfrom a third excitation flash will be greater than the time to the firstmeasurement point resulting from the second excitation flash, andlikewise for any subsequent excitation flashes.

Effectively the time to the first phosphorescence measurement pointsresulting from the first and subsequent excitation flashes arerespectively offset such that the first measurement point resulting fromthe second excitation flash follows the first measurement pointresulting from the first excitation flash, and the first measurementpoint resulting from a third excitation flash follows the firstmeasurement point resulting from the second excitation flash, andlikewise for any subsequent excitation flashes.

The controlled time periods may be such that the interleaved data pointsare separated by equal time intervals. Alternatively the controlled timeperiods may be such that the interleaved data points are separated byunequal time periods. That is, this third aspect of the inventionencompasses an operator being able to decide the particular timeintervals that will exist between successive interleaved data pointswhich define the phosphorescence decay characteristic. These particulartime intervals may be equal or unequal and varied, as the operatordetermines.

FIG. 8 illustrates a phosphorescence decay characteristic 200 (intensityvs. time) for a sample for which it is desired that,measurement data atpoints 202-215 be collected to define the characteristic. Sampleintegration periods for the data points 202-216 are shown at time-lines(b), (c), (d) and (e). The time intervals shown along time-line (a)represent the time for the data of a measurement point to be transferredto the computer. This time is greater than the spacing between the datapoints 202-21), thus the computer cannot collect the data of all thedesired measurement points in one pass.

According to the invention, several decay scans are performed and thedata from each are interleaved in a correct time sequence to derive thephosphorescence decay characteristic. The measurement regime is undercontrol of computer 130 which keeps track of all the delay/emission datapoints required to define the phosphorescence decay characteristic.Following an excitation flash, the data acquisition electronics42-124-126-128-130 completes collection of the first datum 202 asrepresrented by time interval at (a), the computer notes the time fromthe excitation pulse, looks for the next unmeasured point 206 after thattime and triggers the data acquisition electronics to collect that pointas well, and so on for the illustrated datums 210 and 214. Thus severaldata points are measured on the one cycle as shown at (b ). The nextexcitation pulse is then triggered, and under the control of thecomputer, the data points 203, 207, 211, 215 are collected, as shown at(c) and so on for as many cycles as are required to collect all thedesired data points, see (d) and (e).

The computer then assembles all the measured data points into thecorrect overall time sequence to create the complete phosphorescencecharacteristic. For example, if measurement at 1 microsecond intervalsis required to define the phosphorescence characteristic and theacquisition of each measurement data point takes 19.5 microseconds, onthe first cycle the emission at 1 microsecond is measured. This data istransferred to the computer at 20.5 microseconds. The next data pointrequired is at 21 microseconds so the computer triggers the electronicsto collect this point. The second point is transferred to the computerat time 40.5 microseconds. The next data point required is at 41microseconds and so the computer triggers the electronics to collectthis point, and so on. On the second cycle, the computer collects datafor times 2 microseconds, 22 microseconds, 42 microseconds etc. In thisexample, for data covering 2 milliseconds, all points can be collectedin 20 cycles instead of 2000 cycles for the prior art approach, reducingthe overall measurement time from 20 seconds to 200 milliseconds.

The time intervals between the data points may vary or be fixed and isnot critical to the invention, which is characterised by the collectionof more than one data point from each cycle and the reassembly of thosedata points into the correct time sequence within the associatedcomputer system.

This third aspect of the invention offers several advantages. The firstis the time saving leading to increased productivity. Some samples havethe characteristic of changing their properties with time or with theamount of excitation light received. The invention reduces both themeasurement and the total integrated amount of excitation light imposedon the sample thereby minimising this source of measurement uncertainty.

In order to obtain good time precision for short duration 15phosphorescence events the duration of the excitation pulse needs to beshort while at the same time delivering a high total 1 light flux to thesample. A xenon flash lamp 20 meets these requirements of short durationand high intensity and is thus a desirable source for such applications.It has however the disadvantage that the light output per flash isvariable. Since the emission signal is proportional to the excitationsignal such variation must be allowed for if accurate results are to beobtained. To this end, in the implementation of the phosphorescencemeasurement method, the excitation flux for each pulse is measured atthe start of each cycle and used to normalise the emission measurementscollected during that cycle. That is, as is known, a dark signal ismeasured for the sample for each cycle, and a dark signal and referencesignal for the reference beam 30 which are used to normalise theresults.

A further variant on this third aspect is to use two or more independentsets of measurement and digitisation electronics operating from the samesignal source and all capable of transmitting the digitised value to theprocessor. In this case the processor initiates a first excitation flashof light and as each measurement time instant is reached it triggers thenext available set of measurement and digitisation electronics toacquire the value. This variant has the advantage of achieving stillshorter data collection times but at the expense of greater electroniccost and complexity. Thus two or more independent sets of digitisationelectronics may be used in conjunction with multiple flashes to givestill greater speeds.

Preferably a reference intensity measurement is taken of everyexcitation flash of light, and the phosphorescence emission intensitymeasurements derived from that flash are ratioed with the reference tocompensate for differences in intensity which may occur between flashes.As is known such compensation may include dark signal measurements beingtaken immediately before an excitation flash and subtracted from themeasured excitation and emission intensities for the ratioing.

The invention in each of its aspects as described herein is susceptibleto variations, modifications and/or additions other than thosespecifically described and it is to be understood that the inventionincludes all such variations, modifications an/or additions which fallwithin the scope of the following claims.

What is claimed is:
 1. A spectrometry instrument and an exchangeableaccessory therefor including a manually operable mechanism for attachingthe exchangeable accessory to the instrument, the mechanism including amanually rotatable camming means associated with one of the accessory orthe instrument, a male member associated with the other of the accessoryor the instrument, the male member having a camming surface which isengageable by the camming means, wherein the accessory is positionableon the instrument in a predetermined location and the camming means ismanually rotatable to engage the camming surface of the male member andthereby lock the accessory on the instrument in the predeterminedlocation, wherein the camming means includes a body having a recessformed therein, the recess having a curved camming surface which,interacts with the camming surface of the male member.
 2. A spectrometryinstrument and an exchangeable accessory therefor as claimed in claim 1wherein the camming surface of the male member is substantiallyspherical, and the recess of the camming means has a substantiallycomplementary shape.
 3. A spectrometry instrument and an exchangeableaccessory therefor as claimed in claim 1 wherein the male member isbiased in a direction towards the instrument or accessory with which itis associated, whereby engagement of the camming means with the cammingsurface of the male member moves the male member against the bias.
 4. Aspectrometry instrument and an exchangeable accessory therefor asclaimed in claim 1 wherein the camming means and male member aresubstantially centrally located on facing surfaces of the instrument andthe accessory.
 5. A spectrometry instrument and an exchangeableaccessory therefor as claimed in claim 1 wherein facing surfaces of theinstrument and the accessory include projections and complementaryrecesses for establishing said predetermined location.
 6. A spectrometryinstrument and an exchangeable accessory therefor as claimed in claim 4wherein the camming means includes a shaft which extends to a peripheralsurface of the instrument or accessory with which the camming means isassociated, and a handle or knob the shaft adjacent said peripheralsurface for facilitating manual operation of the camming means.
 7. Aspectrometry instrument and an exchangeable accessory therefor asclaimed in claim 1 wherein the camming means is associated with theaccessory and the male member is associated with the instrument.
 8. Aspectrometry instrument and an exchangeable accessory therefor asclaimed in claim 1 including an electrical circuit for identifying anyone of a plurality of exchangeable accessories that are connectable tothe instrument, the electrical circuit including a voltage source andmeans for generating an identifying voltage therefrom, wherein eachaccessory includes at least one circuit element such that connection ofan accessory to the instrument alters the identifying voltage to a valuethat is uniquely dependent upon the accessory that is connected to theinstrument.
 9. A spectrometry instrument and an exchangeable accessorytherefor as claimed in claim 8 wherein the means for generating anidentifying voltage from the voltage source is a voltage divider.
 10. Aspectrometry instrument and an exchangeable accessory therefor asclaimed in claim 8 wherein the voltage source and means for generatingan identifying voltage provide an open electrical circuit such that inthe absence of an accessory the identifying voltage becomes the voltageof the voltage source thereby identifying the absence of an accessory.11. A spectrometry instrument and an exchangeable accessory therefore asclaimed in claim 10 wherein the at least one circuit element of anaccessory completes said open circuit.
 12. A spectrometry instrument andan exchangeable accessory therefor as claimed in claim 11 wherein the atleast one circuit element of an accessory is a circuit link thatconnects the electrical circuit of the instrument to the instrumentground or a predetermined one of a plurality of voltages of theinstrument, depending on the accessory.
 13. A spectrometry instrumentand an exchangeable accessory therefor as claimed in claim 10 whereinthe accessory includes a plurality of circuit elements that completesaid open electrical circuit.
 14. A spectrometry instrument and anexchangeable accessory therefor as claimed in claim 8 wherein the atleast one circuit element is a winding of an electrical motor of anaccessory, whereby the identifying voltage is altered to a value thatuniquely identifies the presence of the electrical motor.
 15. Aspectrometry instrument and an exchangeable accessory therefor asclaimed in claim 8 wherein the electrical circuit of the instrumentadditionally provides for recognition of the presence of an electricalmotor in an accessory.
 16. A spectrometry instrument and an exchangeableaccessory therefor as claimed in claim 15 wherein the winding of themotor completes an electrical circuit of the instrument that includes avoltage divider whereby a logic high signal is generated by the presenceof the winding and a logic low signal in the absence of the winding. 17.A spectrometry instrument and an exchangeable accessory therefor asclaimed in claim 8 including a computer having a microprocessor forreading the identifying voltage and thereby identifies the accessoryconnected to the instrument.
 18. A spectrometry instrument and anexchangeable accessory therefor as claimed in claim 17 wherein thecomputer is programmed to automatically load programs for operating theinstrument in measurement regimes involving the accessory which isconnected to the instrument.